Producing renewable clean energy is one of the most profound challenges of the 21st century. Most of the world's current energy supplies come from sunlight converted to chemical energy by photosynthesis in plants. Photosynthesis begins with light striking a light harvesting complex in Photosystem II, creating a charge-separated excited state where an electron is promoted to a higher energy level. Water oxidation catalyzed by an oxygen evolving complex (OEC) replenishes the hole derived from the charge-separated excited state. This process causes the release of four equivalents of H+ per water molecule. Some of the generated protons are used in a proton gradient, which the plant employs to store energy by synthesizing ATP. The high energy electrons move along an electron transport chain and are eventually used to reduce the remaining protons from the water oxidation process.
A central thrust of current energy research focuses on artificial photosynthesis, namely synthetic light-harvesting, water-based fuel producing systems. Despite the intense global efforts to develop viable biological water splitting systems for energy production, breakthroughs are needed in efficiency and stability of the three operational units in such a composite system: the sensitizer for light absorption (charge separation), the catalyst for the proton-reduction half-reaction (2H++2e−→H2), and the catalyst for the water-oxidation half-reaction (H2O→O2+4H++4e−).
Developing efficient and robust synthetic water-oxidation catalysts (WOCs) has proven particularly challenging, because this half-reaction is the most thermodynamically demanding part of water splitting. An effective WOC must be capable of water-oxidation at a potential minimally above the thermodynamic value (1.23 V at pH 0) to minimize energy losses. Furthermore, it should do this as quickly as possible to maximize space-time-yields of the overall device and to avoid charge accumulation (leading to corrosion) and recombination (energy loss). Finally, it has to be as robust as possible with regards to trace contamination, pH variations, current densities, and long-term operation to enable real-world application.
Both heterogeneous and homogeneous WOCs have been investigated. Heterogeneous WOCs consisting of bulk metal oxides, metal oxide layers or nanoparticles on conducting substrates generally have the advantages of ease of interface with electrode systems and good oxidative stability, but their synthesis is often laborious and leads to ill-defined materials that are hard to characterize and thus optimize. Furthermore, since only the surface layer of the electrode material is able to engage in the water-oxidation reaction at the interface with the aqueous solution their efficiency is inherently limited.
Homogeneous WOCs can be more amenable to spectroscopic, crystallographic, physiochemical, and computational investigation, and thus may be more readily optimized on a rational basis. Additionally, as each individual molecule of a homogeneous catalyst is capable of doing solution chemistry the efficiency of the process is intrinsically higher, thereby reducing the amount of catalyst needed. However, most of the known homogeneous WOCs that have been investigated are thermodynamically unstable with respect to oxidative degradation. As a result, no system has yet succeeded to combine the high efficiency of homogeneous WOCs with the durability and ease of application of heterogeneous ones. Such a catalyst system could be active in the reverse oxygen reduction reaction and electrode-driven hydrocarbon oxidations, the two key reactions of carbon fuel cells
An alternative strategy to supply society with sustainable fuels, which circumvents the H2 storage issues that are still problematic for a solar hydrogen economy, is the use of simple liquid hydrocarbons such as short chain alcohols, ethers, and lactones as energy carriers. As both biogenic and synthetic substrates could be used in such a scheme, a ‘methanol economy’ would serve as a flexible technology facilitating the transition to more sustainable energy supply. One of the key issues with this technology is, however, the energy-out part, i.e. the controlled total oxidation of the organic energy carrier in a low-temperature fuel cell. In a fuel cell, both the anode reaction (substrate oxidation to release protons & electrons) and the cathode reaction (O2 reduction to H2O) need effective electro-catalysts to maximize overall efficiency as explained above for the reverse case of water splitting.
Electrochemical total hydrocarbon oxidation to CO2 at low temperatures is particularly challenging, and molecularly defined catalysts for this reaction are extremely rare. Preliminary evidence suggests that the catalysts also perform electrochemical hydrocarbon oxidation, which make them interesting for low-temperature direct carbon fuel cell applications. Furthermore, preliminary evidence suggest that the materials also catalyze the reverse reaction, electrochemical oxygen reduction, which is the ubiquitous cathode reaction in any type of fuel cell (including hydrogen fuel cells).
There exists a need for water oxidation catalysts that can split water at near-thermodynamic or extremely low overpotentials with a high turnover frequency for long periods of time without degrading, particularly using low cost materials or small amounts of highly active materials.
There exists a need for C—H oxidation catalysts that can selectively oxidize C—H bonds using an applied potential. A heterogeneous or surface bound molecular system is advantageous here because the reactants, products, and catalyst need to be separated at a later point, so having a catalyst that doesn't need to be separated is beneficial.
There is also a need for oxygen reduction reaction (ORR) catalysts that catalyze oxygen reduction at fuel cell cathodes, particularly at as low an overpotential as possible. Efficient ORR catalysts would dramatically increase the efficiency of fuel cells.
Therefore, it is an object of the invention to provide water oxidation catalysts that can split water at near-thermodynamic or extremely low overpotentials with a high turnover frequency for long periods of time without degrading, particularly using low cost materials or small amounts of materials and methods of making and using thereof.
It is also an object of the invention to provide C—H oxidation catalysts that can selectively oxidize C—H bonds using an applied potential, particularly heterogeneous or surface bound molecular catalysts which allow for straight forward separation of the reactants, products, and catalyst, and methods of making and using thereof.
It is also an object of the invention to provide catalysts for other electrochemical oxidation reactions, such as the oxidation of chloride to chlorine, bromide to bromine, and ammonia to nitric acid, particularly when using a surface bound molecular catalyst that allows for a lower overall energy cost in production of the product chemical.
It is also an object of the invention to provide oxygen reduction reaction (ORR) catalysts that catalyze oxygen reduction at fuel cell cathodes, particularly at as low an overpotential as possible and methods of making and using thereof.